The present invention relates to electrographic sensors, and particularly contact sensors.
Contact sensors, and particularly touchscreens, are becoming a popular input device for computers. Touchscreens allow people who have never used a computer before to easily interact with the computer. They are finding use in cars, airports, factories, shopping malls, hospitals, training centers, banks, grocery stores, libraries, and pharmacies. Common applications are automatic bank tellers, point of sale systems, and information systems found in airports for providing directions to various locations or information regarding local hotels.
One common type of touchscreen is an electrographic touchscreen or sensor of the type described in U.S. Pat. No. 4,220,815, which is incorporated herein by this reference. Such a screen typically comprises a first sheet of flexible material capable of being energized to establish an electrical potential. The sensor also has a second sheet capable of being energized to establish an electrical potential in juxtaposition with the first sheet. By pressing on the first sheet with a stylus or finger, selected portions of the two sheets come together, thereby generating an electrical signal corresponding to the locus of the portion of the flexible sheet pressed. In order to keep the two sheets separated when they are not pressed, a plurality of discrete insulating buttons or islands are provided. The islands are sized and located so pressure on the first sheet results in the two sheets contacting.
A problem of contact sensors is that the two sheets can occasionally remain in contact when the pressure is removed, i.e., the two sheets stick together. This sticking renders the device on which the sensor is being used inoperative. Such sticking can result from environmental conditions, such as high temperature and/or humidity, manufacturing defects, and the cohesive forces of the material selected.
Sticking is a serious and costly problem. Often the entire sensor needs to be replaced when sticking occurs. Moreover, the entire machine using the sensor is down and inoperative until the problem has been solved. Further, much user ill can occur when a computer system does not operate because of a defective touchscreen.
Much scientific effort has been directed to this problem. Among the solutions considered have been changing the size and/or material of the insulating islands, roughening the surface of the various sheets, changing the materials of the sheets, and providing special coatings on the sheets. None of these solutions has been totally satisfactory, either not working, or being expensive or difficult to implement.
Accordingly, there is a need for a touchscreen that has all the attributes of commercially available touchscreens, namely ease of use, without the sticking problem that plagues many of the touchscreens currently on the market.